1. Field of the Invention
This invention relates to electronic circuits, and more particularly, to voltage controlled oscillators (VCOs).
2. Description of the Related Art
Phase-locked loops (PLLs) are commonly used for the generation of oscillating signals in a wide variety of electronic circuits. For example, PLLs may be used in digital circuits for clock generation and reproduction, in radio transmitter and receiver circuits for generation of signals at radio frequencies, and so forth.
A typical PLL includes a phase detector coupled to receive a reference signal, a low pass filter coupled to the output of the phase detector, and a voltage controlled oscillator (VCO). The output of the VCO may provide the output signal of the PLL, and may also be fed back to an input of the phase detector via a feedback loop. In some instances, the output of the VCO may be divided before being provided to the phase detector. The phase detector is configured to compare the reference signal and the signal received from the feedback loop, and adjust its output voltage accordingly.
One type of VCO commonly used in PLLs utilizes an LC (inductive-capacitive) tank circuit. During operation of a VCO based on an LC tank circuit, an electric current may alternate between the inductor and the capacitor at a resonant frequency of the circuit. To achieve a low-noise VCO output, the oscillation signal amplitude must be sufficiently large such that the oscillation frequency is more resilient to noise sources. However, the amplitude cannot be excessive as the large amplitude swing will otherwise cause varactor capacitance averaging and consequently degrade the oscillator tuning range.